This is why I should become an academic. Link to pdf of paper by Simons, D. J. & Levin, D. T. (1998): "Failure to detect changes to people during a real-world interaction." The researchers have an actor stop passersby and ask for directions, then actors depicting workmen pass between them with a large door, and the original actor is swapped with someone else. The directions-givers don't have a clue!